high frequency intermittent noise

James, one other thing to try, use an extension cord if you must, but maybe you have other AC lines which are less noisy, or perhaps a place in your apartment where the noise pickup is less?

Just throwing out ideas that are cheap. :)

But yeah, I like Furman for noise and value (especially if you can find them on sale!)

@erik_squires The Furman looks promising.  The P 2400 IT looks like the way to go.

@james1969 ,

The noise is primarily during the day and goes into the evenings, weekends too, early mornings it does not appear.

"early mornings it does not appear." Never? Have you checked the time in the morning the noise stops and the length of time the noise is not present? Is there a consistent pattern early every morning. The noise stops at __ am and starts up again at __ am
Same times every day?

I have a cell phone tower directly across the street from my building in Manhattan.

But the sound is not 24/7.

I live in a pre-war mid-rise building.

How many stories high?

Is there a building maintenance person on duty, someone you could speak to about the noise? If so before you talk to them see if you can pin down the time in the early morning the noise stops and the time it starts back up again.

@jea48 The noise does present itself 24 hours a day randomly, it is only the early mornings ~ 4am to 11am it is not very frequent, but it does appear.  The building is 15 stories high.  I have already had 2 different electricians in, and they did not do anything.

As I was writing my last post I realized another important fact:

• The subway runs under my building.
  

I just noticed the noise appearing when I listened for a train to go by.  There seems to be a correlation here, perhaps it's the subway system:  when I hear the subway go by, the noise is apparent.  That would also make sense in that the subway does not run as frequent in the early hours of the day.